The invention relates generally to high intensity discharge lamp luminaires and, more particularly, to luminaires including metal halide lamps that are susceptible to non-passive failure.
Metal halide gas discharge lamps have a number of desirable characteristics, including a good color balance suitable for indoor lighting (in contrast to mercury vapor and sodium lamps), and relatively efficient operation. They are widely used in many applications such as industrial lighting and sports lighting.
Metal halide lamps however can present a potential ignition problem. High intensity discharge lamps in general include a quartz or ceramic arc tube with a gaseous fill, and a pair of tungsten electrons located inside the arc tube at opposite ends. An arc between the electrodes emits visible light. In the case of a metal halide high intensity discharge lamp, the pressure inside the arc tube may reach 440 psi (30 bar), and the temperature may reach 1100xc2x0 C. Metal halide lamps are subject to non-passive failure whereby hot particles of quartz or ceramic arc tube and tungsten electrode materials fall as hot debris, potentially igniting flammable objects below. Some metal halide lamp luminaires include a containment barrier for hot debris in the event of non-passive failure.
Thus, one general type of metal halide lamp luminaire takes the form of a lamp enclosure including a reflector having an open end oriented generally downwardly, and a transparent closure covering the open end. The transparent closure is conventionally referred to as a lens or refractor. As employed herein, the conventional term xe2x80x9clensxe2x80x9d is not intended to be limited to a transparent closure with refractive qualities. However, in most cases, in order to produce a controlled lighting pattern, the lens has a prismatic interior (upper) surface, a prismatic exterior (lower) surface, or both, for reflecting and refracting light from the lamp.
In many respects, a good lens material is a transparent polymeric material such as acrylic polymer. Acrylic polymer is lightweight, transparent, and readily molded. It is relatively resistant to yellowing, particularly if an ultraviolet filter is employed to reduce the amount of ultraviolet radiation from the lamp reaching the acrylic resin material itself.
A disadvantage, however, of acrylic resin is that it is both flammable and thermoplastic, and subject to ignition and even melt-through by hot debris in the event of non-passive failure of a metal halide lamp.
There is an Underwriters Laboratory standard on containment, number UL1572, which has been updated to UL1598. In a containment barrier test pursuant to UL1572, a sample section of acrylic lens material is heated up to 88xc2x0 C., which is the maximum expected nominal use temperature for one particular manufacturer. A surface located 12 inches (30.48 cm) below the acrylic lens sample is covered by a layer of dry absorbent cotton that is 0.25 inch (6.35 mm) thick. Quartz particles heated up to 1100xc2x0 C. are dropped on to the acrylic lens. In most cases, the acrylic lens ignites, and the particle sinks into the acrylic lens. Failure is defined as the cotton being ignited by flaming drips of plastic material or any quartz particle that penetrates the acrylic lens material and falls on the cotton.
In order to provide sufficient containment, acrylic lenses are typically made relatively thick, for example 0.110 inches (2.794 mm) as a minimum, which has the disadvantages of adding to the cost and increasing the loss of light.
Another approach to containment which has been employed in the past is to place a layer of fiberglass on the upper refractor surface. In that prior approach, a circular piece of fiberglass sheet is cut out and attached to the upper relatively flat surface of the refractor or lens. The fiberglass sheet separates the acrylic from the hot particles, but reduces the light output of the luminaire by over 10%, and changes the light distribution pattern.
Yet another approach is to employ a transparent closure which is made of glass. While not subject to combustion, glass has disadvantages in that it is relatively heavy, is subject to shattering, and it is difficult to form prismatic surfaces having sharp edges in the case of a glass lens. A hybrid prior art approach is to employ a piece of glass above an acrylic refractor. In addition to the disadvantage of added cost, luminaire light output is reduced.
It is therefore seen to be desirable to improve the containment of hot debris in the event of non-passive failure of a metal halide gas discharge lamp in a luminaire including an acrylic lens.
It is further seen to be desirable to reduce the cost of an acrylic lens or refractor, and to increase luminaire light output, by decreasing the thickness of the acrylic lens.
In an exemplary embodiment of the invention, a luminaire comprises a lamp enclosure including a reflector having an open end oriented generally downwardly, and a transparent closure made of a combustible polymeric material covering the open end. A high intensity discharge lamp is contained within the enclosure. The high intensity discharge lamp includes an arc tube and is subject to non-passive failure whereby hot debris such as particles of arc tube material fall on to an interior, upper surface of the lens. The interior, upper surface of the lens is ignition-resistant and, in exemplary embodiments, comprises a coating.
Quite surprisingly, very thin coatings of materials such as silicone hardcoat, or a combined coating of a silicon oxynitride having a composition SiOxNy over a thin layer of silicone hardcoat, are highly effective. One expected result might be that a thin coating would serve as an oxygen barrier, and that a hot quartz particle would sink into the acrylic, but without an immediate flame. However, quite surprisingly, not only is there no flame, but hot debris particles do not sink into the acrylic. The quartz particles simply sit on top of the coated acrylic, and in some cases xe2x80x9cdancexe2x80x9d around, perhaps due to Leidenfrost phenomenon.
These very thin coatings do not adversely affect the optical characteristics of the lens and, in fact, can provide advantages such as scratch resistance and ultraviolet absorption. Lens thickness can be decreased, for a reduction in cost and an increase in light output.